The present invention relates to an electromagnetic actuator for driving a hammer of an impact printer or the like, and more particularly to improvements of an electromagnetic actuator for obtaining releasibility of an armature attracted toward a core by the magnetic attractive force of a solenoid, at the time of the shutting off of a current supplied to the solenoid.
Conventionally, electromagnetic actuators, in which a solenoid having a coil of wire wound around a core is used as an electromagnet for generating a magnetic attractive force, are used in various industrial applications. Typical applications include, for instance, electric relays and electromagnetic clutches, in which the solenoid is used to attract a movable armature.
Printers that are output apparatus for computers or the like are another known field of application of the electromagnetic actuators.
A conventional impact printer is generally arranged such that hammers for respective columns are arranged in face-to-face relationship with a print drum along which recording paper is guided, and when the print drum is rotatively driven to predetermined printing positions, the hammers which are previously held at urge-accumulated positions by electromagnetic actuators against the force of printing springs are released and impacted against the print drum, thereby the hammer prints a desired type on the recording paper.
More specifically, the urge-accumulated state of the hammer is held by the armature of the electromagnetic actuator, and an exciting current is supplied to the electromagnetic actuator with the printing drum set at a predetermined printing position and at a predetermined timing set on the bases of a printing command, whereupon the armature releases the hammer so as to effect the above-described impacting operation.
FIG. 6 shows a cross-sectional view of a conventional electromagnetic actuator. As shown, a solenoid 10 comprises a coil 10a, a bobbin 10b, and a core 10c, the coil 10a being provided on the bobbin 10b and the core 10c being inserted in the bobbin 10b.
A spacer 13 is secured to an end surface of the core 10c. One end of a yoke 11 for introducing the magnetic flux of the solenoid 10 is secured to the solenoid 10.
An armature 12 is pivotally supported at the other end of the yoke 11 in such a manner as to be rotatable at a slit or indent (hereafter referred to as the slit) 11a. In addition, a return spring 15 is stretched in a state of tension between one end of a baseplate 14 to which the yoke 11 is secured and an end 12a of the armature 12. This return spring 15 holds the armature 12 in such a manner as to be rotatable with respect to the yoke 11 and urges the armature 12 in the direction of being released from the core 10c.
As a current is supplied to the coil 10a of the solenoid 10, the armature 12 is attracted toward the core 10c against the return force of the return spring 15, and is hence attracted to an end surface of the core 10c via a spacer 13.
This spacer 13 is formed of a nonmagnetic material and is adapted to prevent the armature 12 thus attracted from directly abutting against the core 10c. This arrangement is provided to prevent a delay in the release of the armature 12 caused by residual magnetism and weaken the attractive force so as to facilitate an immediate release thereof at the time of a shut-off of a current, thereby preventing faulty printing attributable to a delay in the release of the armature.
However, the conventional spacer merely forms a nonmagnetic gap between the armature and the core, so that it is difficult to positively obtain sufficient releasibility of the armature. Hence, it has been difficult to positively prevent faulty printing attributable to a delay in the release of the armature.
In other words, if the conventional spacer alone is used which is formed of a nonmagnetic material, the releasing of the armature cannot be effected speedily. If an attempt is made to increase the urging force of the armature returning spring so as to improve the releasibility of the armature, a problem is encountered in that the attractive force of the electromagnetic actuator must be increased.
Consequently, as described above, with impact printers or the like, since the releasing timing of the armature can be unstable, there are cases where erroneous printing or the like occurs, including double printing by the impacted hammer.